1. Field of the Invention
The present invention relates to a liquid crystal display device as a computer monitor and a liquid crystal display device for displaying video images and, more particularly, to a liquid crystal display device having desirable viewing angle characteristics.
2. Description of the Related Art
Various display modes have been proposed for improving the viewing angle of a liquid crystal display device. Typical examples of such display modes include: {circle around (1)} IPS (In-Plane Switching) mode which uses a transverse electric field to move the liquid crystal molecules in parallel with the substrate surface; {circle around (2)} a liquid crystal display device in which the liquid crystal molecules are oriented substantially vertically to the substrate surface in the absence of an applied voltage, and the liquid crystal molecules in different regions are inclined in respectively different directions in the presence of an applied voltage (Japanese Laid-Open Publication No. 7-28068); {circle around (3)} a liquid crystal display device in which the liquid crystal molecules are oriented substantially horizontally to the substrate surface in the absence of an applied voltage, and the liquid crystal molecules in different regions rise in respectively different directions in the presence of an applied voltage, thereby improving the viewing angle of the device (Japanese Laid-Open Publication No. 10-3081); and {circle around (4)} a liquid crystal display device of a normally black mode producing a black display in the absence of an applied voltage, in which an optical compensator is used to improve the viewing angle of the device (Japanese Laid-Open Publication No. 5-289097).
In the IPS mode {circle around (1)}, however, it is necessary to provide a plurality of opaque electrodes in a pixel, thereby reducing the aperture ratio and thus the transmissivity (brightness) of the device. The liquid crystal display device {circle around (2)} disclosed in Japanese Laid-Open Publication No. 7-28068 employs a liquid crystal material having a negative dielectric anisotropy (n-type liquid crystal material) and a substrate which has been subjected to a vertical alignment treatment In such a case, the liquid crystal injection process takes a period of time twice as long as that for a device which employs a p-type liquid crystal material having a positive dielectric anisotropy and a substrate which has been subjected to a horizontal alignment process, thereby reducing the production efficiency. The liquid crystal display device {circle around (3)} disclosed in Japanese Laid-Open Publication No. 10-3081 employs transparent electrodes provided respectively on the upper and lower substrates to drive the liquid crystal molecules, thereby avoiding the reduction in transmissivity as in the IPS mode. Moreover, The liquid crystal display device {circle around (3)} employs a liquid crystal material having a positive dielectric anisotropy and a substrate which has been subjected to a horizontal alignment treatment, thereby also avoiding the reduction in production efficiency as in the device {circle around (2)}. However, the device {circle around (3)} has viewing angle characteristics that are inferior to those of the device {circle around (2)}. Particularly, the device {circle around (3)} has asymmetric gray scale characteristics in the vertical direction along the display plane.
Referring to FIG. 55, the liquid crystal display device {circle around (4)} disclosed in Japanese Laid-Open Publication No. 5-289097 includes a liquid crystal panel 4, a birefringence anisotropy compensation panel 3 provided optically continuously on the liquid crystal panel 4 for optically compensating the birefringence anisotropy of the liquid crystal panel 4 along the plane thereof, a viewing angle dependency compensation panel 2 provided on the birefringence anisotropy compensation panel 3, and a pair of polarizers 1 and 5 interposing the panels 2, 3 and 4 therebetween, so that the absorption axes (1.1) and (5.1) are perpendicular to each other. The birefringence anisotropy compensation panel 3 is arranged so that the optical axis (3.1) or (3.2) (or rubbing direction) thereof is parallel to the substrate surface of the liquid crystal panel 4 and perpendicular to the optical axis (4.1) or (4.2) (or rubbing direction) of the liquid crystal panel 4. The viewing angle dependency compensation panel 2 is arranged so that the optical axis (2.1) (or rubbing direction) thereof is perpendicular to the substrate surface of the liquid crystal panel 4. The device {circle around (4)} provides a certain level of improvement in the viewing angle characteristics thereof by employing the viewing angle dependency compensation panel 2. However, the contrast of the device {circle around (4)} is reduced as the viewing angle is inclined or shifted from a direction normal to the display plane. Thus, the viewing angle characteristics of the device {circle around (4)} are not sufficiently desirable. Moreover, it is difficult to stably obtain a uniform orientation and transmissivity across the display plane of the liquid crystal panel 4 of the device {circle around (4)} in the presence of an applied voltage.
According to one aspect of this invention, a liquid. crystal display device includes: a first transparent substrate and a second transparent substrate; a liquid crystal layer interposed between the first and second substrates, the layer being made of a nematic liquid crystal material having a positive dielectric anisotropy; a first electrode and a second electrode provided on the first and second substrates, respectively, for applying an electric field substantially vertical to the first and second substrates across the liquid crystal layer; and a first polarizing plate and a second polarizing plate each provided on an outer side of respective one of the first and second substrates, the first and second polarizing plates being arranged in a crossed Nicols arrangement. The liquid crystal layer in each pixel region includes at least a first domain and a second domain in which liquid crystal molecules are oriented in different orientations. A first phase difference compensator having a positive refractive index anisotropy is provided between the first polarizing plate and the first substrate, and a second phase difference compensator having a positive refractive index anisotropy is provided between the second polarizing plate and the second substrate, so that phase-delay axes of the first and second phase difference compensators are parallel to a substrate surface and to each other, and substantially perpendicular to a phase-delay axis of the liquid crystal layer in the absence of an applied voltage. At least one third phase difference compensator is provided between the first polarizing plate and the first phase difference compensator or between the second polarizing plate and the second phase difference compensator. A refractive index ellipse of the third phase difference compensator has three main axes a, b and c, and refractive indexes of na, nb and nc along the main axes a, b and c, respectively, wherein a relationship nc greater than na greater than nb holds, with the main axis a and the main axis b lying in a plane parallel to the substrate surface, the main axis c being parallel to a direction normal to the substrate surface, and the main axis a being perpendicular to a polarization axis of one of the polarizing plates which is adjacent to the phase difference compensator. The first, second and third phase difference compensators compensate for a refractive index anisotropy of the liquid crystal molecules of the liquid crystal layer which are in a substantially horizontal orientation with respect to the substrate surface in the absence of an applied voltage.
In one embodiment of the invention, where a retardation value of the liquid crystal layer is d1cxc2x7xcex94n, an in-plane retardation of the third phase difference compensator is dxc2x7(naxe2x88x92nb), and a retardation along a thickness direction thereof is dxc2x7(naxe2x88x92nc); parameters RL and NZ are defined as follows
RL=dxc2x7(naxe2x88x92nc)/(d1cxc2x7xcex94n), and
NZ=(naxe2x88x92nc)/(naxe2x88x92nb);
two of the third phase difference compensators are provided respectively between the first polarizing plate and the first phase difference compensator, and between the second polarizing plate and the second phase difference compensator, with a sum of RL values of the two third phase difference compensators being defined as RLsum; then,
0xe2x89xa6|RLsum|xe2x89xa62; and
each of the third phase difference compensators satisfies log(|NZ|)xe2x89xa72.0xc2x7|RL|xe2x88x921.2, where RL less than 0 and NZ less than 0.
In one embodiment of the invention, the RL value and the NZ value of one of the two third phase difference compensators are equal to the RL value and the NZ value, respectively, of the other one of the two third phase difference compensators.
According to another aspect of this invention, a liquid crystal display device includes: a first substrate and a second substrate at least one of which is transparent: a liquid crystal layer interposed between the first and second substrates, the layer being made of a nematic liquid crystal material having a positive dielectric anisotropy; a first electrode and a second electrode provided on the first and second substrates, respectively, for applying an electric field substantially vertical to the first and second substrates across the liquid crystal layer; a first polarizing plate and a second polarizing plate each provided on an outer side of respective one of the first and second substrates, the first and second polarizing plates being arranged in a crossed Nicols arrangement; and a phase difference compensator. The liquid crystal layer in each pixel region includes at least a first domain and a second domain in which liquid crystal molecules are oriented in different orientations. The phase difference compensator compensates for the refractive index anisotropy of the liquid crystal molecules in a substantially horizontal orientation with respect to the surfaces of the first and second substrates.
In one embodiment of the invention, the first and second substrates are both transparent, and the phase difference compensator comprises a first phase difference compensator provided between the first substrate and the first polarizing plate and a second phase difference compensator provided between the second substrate and the second polarizing plate.
In one embodiment of the invention, the first and second phase difference compensators each have a positive refractive index anisotropy, and phase-delay axes of the first and second phase difference compensators are substantially parallel to each other and substantially perpendicular to a phase-delay axis of the liquid crystal layer in the absence of an applied voltage.
In one embodiment of the invention, a third phase difference compensator is further provided between the first phase difference compensator and the first polarizing plate. The third phase difference compensator has a positive refractive index anisotropy. A phase-delay axis of the third phase difference compensator is substantially perpendicular to the first and second substrates.
In one embodiment of the invention, a fourth phase difference compensator is further provided between the second phase difference compensator and the second polarizing plate. The fourth phase difference compensator has a positive refractive index anisotropy. A phase-delay axis of the fourth phase difference compensator is substantially perpendicular to the first and second substrates.
In one embodiment of the invention, a fifth phase difference compensator is provided between the first phase difference compensator and the third phase difference compensator. A sixth phase difference compensator is provided between the second phase difference compensator and the fourth phase difference compensator. The fifth and sixth phase difference compensators each have a positive refractive index anisotropy. A phase-delay axis of the fifth phase difference compensator is substantially perpendicular to a polarization axis of the first polarizing plate. A phase-delay axis of the sixth phase difference compensator is substantially perpendicular to a polarization axis of the second polarizing plate.
In one embodiment of the invention, directors of the liquid crystal molecules in the first and second domains in the middle of the liquid crystal layer along a thickness direction thereof rise in respective directions which are different from each other by about 180xc2x0. The directions are at about 45xc2x0 with respect to the polarization axis of each of the first and second polarizing plates.
In one embodiment of the invention, the liquid crystal molecules in the first and second domains are in a horizontal orientation.
In one embodiment of the invention, the liquid crystal molecules in the first and second domains are in a twist orientation.
In one embodiment of the invention, pre-tilt angles of the liquid crystal molecules on the first and second substrates in the first domain are different from those in the second domain.
In one embodiment of the invention, pre-tilt angles of the liquid crystal molecules on the first and second substrates in the first domain are different from those in the second domain.
In one embodiment of the invention, the liquid crystal layer in each pixel region includes a plurality of the first domains and a plurality of the second domains, the number of the first domains being the same as the number of the second domains.
In one embodiment of the invention, a total area of the first domains is equal to that of the second domains.
Thus, the invention described herein makes possible the advantage of providing a liquid crystal display device having desirable viewing angle characteristics without sacrificing production efficiency and transmissivity.